I don't understand your question. In particular, I don't understand what you mean by "reactions [...] not at equilibrium". Can you please give a specific example?

sorry , I was in a hurry XD
I meant if a body is placed on a smooth surface ,the surface exerts a reaction on the body and so does the body so why are they not at equilinrium , I know its because of the weight mg , I know , but I can't understand why , I mean the 2 reactions are enough bec. According to Newton) 3rd law , every action has a reaction equal in magnitude but opposite in direction , unless the values of reactions are not equal ?

I still cannot make sense of your question because "if a body is placed on a smooth surface ,the surface exerts a reaction on the body and so does the body" (assuming the surface is horizontal) then we do have an equilibrium situation. As long as all force vectors cancel, so there is no net force, then we have equilibrium. I also do not understand "unless the values of the reactions are not equal". That is certainly NOT part of Newton's 3rd law.

Staff: Mentor

You still haven't said what you mean by "not at equilibrium." Like HallsofIvy, I don't understand what you're really asking about.

Right now, my coffee cup is resting on the table next to my elbow. The table exerts an upwards force on the coffee cup. The coffee cup exerts a downwards force on the table. These two forces are equal in magnitude and opposite in direction, according to Newton's Third Law. The coffee cup is also in equilibrium, because it's stationary (not accelerating).

Note however, that the reason why the coffee cup is in equilibrium is not because the two forces named above add up to zero, because those two forces act on different objects. (Maybe this is what is confusing you?)

The coffee cup is in equilibrium because the forces that act on it add up to zero. One force is the upwards force exerted by the table. The other force is the downwards gravitational force exerted by the earth. These two forces add up to zero, therefore the coffee cup is in equlibrium.